MicroRNAs (miRNAs) are small non-coding RNAs that participate in the spatiotemporal regulation of messenger RNA and protein synthesis. Aberrant miRNA expression leads to developmental abnormalities and diseases, such as cardiovascular disorders and cancer; however, the stimuli and processes regulating miRNA biogenesis are largely unknown. The transforming growth factor beta (TGF-beta) and bone morphogenetic protein (BMP) family of growth factors orchestrates fundamental biological processes in development and in the homeostasis of adult tissues, including the vasculature. Here we show that induction of a contractile phenotype in human vascular smooth muscle cells by TGF-beta and BMPs is mediated by miR-21. miR-21 downregulates PDCD4 (programmed cell death 4), which in turn acts as a negative regulator of smooth muscle contractile genes. Surprisingly, TGF-beta and BMP signalling promotes a rapid increase in expression of mature miR-21 through a post-transcriptional step, promoting the processing of primary transcripts of miR-21 (pri-miR-21) into precursor miR-21 (pre-miR-21) by the DROSHA (also known as RNASEN) complex. TGF-beta- and BMP-specific SMAD signal transducers are recruited to pri-miR-21 in a complex with the RNA helicase p68 (also known as DDX5), a component of the DROSHA microprocessor complex. The shared cofactor SMAD4 is not required for this process. Thus, regulation of miRNA biogenesis by ligand-specific SMAD proteins is critical for control of the vascular smooth muscle cell phenotype and potentially for SMAD4-independent responses mediated by the TGF-beta and BMP signalling pathways.

The TGF-beta/activin/BMP superfamily of growth factors signals through heteromeric receptor complexes of type I and type II serine/threonine kinase receptors. The signal originated by TGF-beta-like molecules appears to be transduced by a set of evolutionarily conserved proteins known as SMADs, which upon activation directly translocate to the nucleus where they may activate transcription. Five SMAD proteins have so far been characterized in vertebrates. These factors are related to the mediator of decapentaplegic (dpp) signalling, mothers against dpp (Mad), in Drosophila and to the Sma genes from Caenorhabditis elegans. Smad1 and Smad2 have been shown to mimic the effects of BMP and activin, respectively, both in Xenopus and in mammalian cells, whereas Smad3 (a close homologue of Smad2) and the related protein DPC4, a tumour-suppressor gene product, mediate TGF-beta actions. We report here that DPC4 is essential for the function of Smad1 and Smad2 in pathways that signal mesoderm induction and patterning in Xenopus embryos, as well as antimitogenic and transcriptional responses in breast epithelial cells. DPC4 associates with Smad1 in response to BMP and with Smad2 in response to activin or TGF-beta. DPC4 is therefore a regulated partner of SMADs that function in different signalling pathways of the TGF-beta family.

Bone morphogenetic protein (BMP) receptors signal by phosphorylating Smad1, which then associates with Smad4; this complex moves into the nucleus and activates transcription. Here we report the existence of a natural inhibitor of this process, Smad6, a longer version of the previously reported JV15-1. In Xenopus embryos and in mammalian cells, Smad6 specifically blocks signaling by the BMP/Smad1 pathway. Smad6 inhibits BMP/Smad1 signaling without interfering with receptor-mediated phosphorylation of Smad1. Smad6 specifically competes with Smad4 for binding to receptor-activated Smad1, yielding an apparently inactive Smad1-Smad6 complex. Therefore, Smad6 selectively antagonizes BMP-activated Smad1 by acting as a Smad4 decoy.

We have identified the 30-zinc finger protein OAZ as a DNA-binding factor that associates with Smads in response to BMP2, forming a complex that transcriptionally activates the homeobox regulator of Xenopus mesoderm and neural development, Xvent-2. OAZ contains a BMP signaling module formed by two clusters of fingers that bind Smads and the Xvent-2 BMP response element, respectively. Previously implicated as a transcriptional partner of Olf-1/EBF in olfactory epithelium and lymphocyte development in the rat, OAZ fulfills this role through clusters of fingers that are separate from the BMP signaling module. The mutually exclusive use of OAZ by the BMP-Smad and Olf pathways illustrates the dual role of a multi-zinc finger protein in signal transduction during development.

Summary The signal transducers of the Transforming Growth Factor β (TGFβ)/Bone Morphogenetic Protein (BMP), the Smads, promote the expression of a subset of miRNAs by facilitating the cleavage reaction by Drosha. The mechanism that limits Smad-mediated processing to a selective group of miRNAs remained hitherto unexplored. In this study, we expand the number of TGFβ/BMP-regulated miRNAs (T/B-miRs) to 20. Interestingly, a majority of T/B-miRs contain a consensus sequence (R-SBE) within the stem region of the primary transcripts of T/B-miRs (pri-T/B-miRs). Here, we demonstrate that Smads directly bind the R-SBE. Mutation of the R-SBE abrogates TGFβ/BMP-induced recruitment of Smads, Drosha, and DGCR8 to pri-T/B-miRs, and impairs their processing, while introduction of R-SBE to unregulated pri-miRNAs is sufficient to recruit Smads and allow regulation by TGFβ/BMP. Thus, Smads are multifunctional proteins which modulate gene expression transcriptionally through DNA binding, and post-transcriptionally by pri-miRNA binding and regulation of miRNA processing.